Does Fasting Simultaneously Change the "Brain" and "Gut Microbiota"? The True Nature of the Internal Network Revealed by Weight Loss Research

When people hear the word "diet," many immediately think of actions like "reducing calorie intake," "increasing exercise," or "cutting out snacks." Of course, there's no doubt that energy balance plays a role in weight management. However, recent studies have begun to show that the human body is too complex to explain weight loss by these factors alone.

The focus this time is on a study introduced by ScienceDaily, which found that "intermittent fasting-style dietary restrictions simultaneously changed brain activity and gut microbiota in obese adults." The study, originally published in Frontiers in Cellular and Infection Microbiology, involved a Chinese research team conducting intermittent energy restriction on obese adults for about two months, tracking changes in weight, blood markers, brain activity, and gut microbiota before and after the intervention.

What makes this study interesting is that it doesn't just look at "whether participants lost weight." The research team examined brain activity using fMRI and analyzed the composition of gut microbiota using stool samples. In other words, they aimed to simultaneously observe how the brain and gut environment were moving behind the numbers on the scale.

The study involved obese adults with a BMI of 28 to 45, with 25 participants ultimately being analyzed. The program was structured to transition from a period of recording usual eating habits to a controlled dietary restriction period, and then to a less controlled dietary restriction period closer to home practice. On fasting days, energy intake was gradually reduced, with strict measures such as setting days with 600 kcal for men and 500 kcal for women.

As a result, participants' weight decreased from an average of about 97.5 kg to about 89.9 kg, a reduction of approximately 7.6 kg or about 7.8%. Changes were also observed in body fat, waist circumference, blood pressure, fasting blood glucose, total cholesterol, LDL cholesterol, and enzymes related to liver function, indicating a broad impact on metabolic and cardiovascular indicators, not just "weight loss."

However, it's important to note that this doesn't mean "intermittent fasting is a panacea." The study had a small number of participants and a short duration. Furthermore, it's not conclusive whether fasting itself changed the brain and gut microbiota, whether weight loss triggered these changes, or whether other factors were simultaneously at play. The research team themselves do not position this as proof of causality.

Still, the reason this study captivates people is clear. We often talk about dieting as a matter of "endurance" or "willpower." However, when looking at brain scans and gut microbiota data together, it suggests that appetite and cravings, or the strong urge to eat, may not simply be a matter of weak willpower but could be born from the relationship between brain circuits and gut microbes.

The study observed changes in brain regions related to appetite, cognitive control, emotion, learning, and sensory processing. Specifically, areas such as the orbitofrontal cortex, putamen, anterior cingulate cortex, and dorsolateral prefrontal cortex were involved. These regions are said to be related to reactions when seeing food, impulse control, reward expectations, and learning behaviors.

If dietary restrictions change the activity of these regions, there may be a biological background for people who feel, "It was tough at first, but I got used to it as I continued," or "My cravings for sweets have weakened" during dieting.

In fact, such experiences are also seen in reactions on social media. In the Reddit intermittent fasting community, there were voices sharing experiences like "I found it easier to control the amount and quality of food since I started" and "My cravings for sweets have decreased" in response to a post introducing the same study. On the other hand, there are cautious reactions like "With a scale of 25 people and 62 days, long-term health effects are unknown" and "We should consider whether it's the effect of fasting or simply calorie restriction."

This divergence in reactions is very healthy because this study does not prove that "intermittent fasting will positively change anyone's brain." Rather, it should be viewed as an exploratory study suggesting that when fasting or dietary restrictions are implemented, not only weight but also the brain and gut microbiota may change together.

There are also intriguing results regarding gut microbiota. The study found that the amount of E. coli decreased at multiple points, while bacteria such as Faecalibacterium prausnitzii, Parabacteroides distasonis, and Bacteroides uniformis temporarily increased. Additionally, a correlation was observed between the increase and decrease of certain gut bacteria and changes in the activity of specific brain regions.

The important word here is "correlation." It's still unknown whether gut microbiota changed the brain, whether changes in the brain altered gut microbiota through eating behavior, or whether dietary content and weight loss affected both. Nonetheless, the results support the concept of the "gut-brain axis," where the gut and brain influence each other bidirectionally.

Gut microbiota may affect metabolism, immunity, and the nervous system through short-chain fatty acids, neurotransmitter-like components, and molecules related to inflammation. Meanwhile, the brain determines what and how much to eat through appetite, stress response, sleep, and behavioral choices. In other words, there's a potential cycle where gut microbiota send signals to the brain, the brain changes eating behavior, and that eating behavior again alters gut microbiota.

Considering this cycle, the difficulty of obesity and weight loss appears differently. Weight gain, strong appetite, inability to quit sweets, and rebound are phenomena that are too crude to dismiss as merely "weak willpower." Appetite is born from the interplay of the brain's reward system, gut environment, blood sugar, hormones, sleep, stress, and social environment. Dieting is difficult not because humans are lazy, but because the body has a powerful homeostasis maintenance system.

On social media, voices resonate with this point. Among those practicing fasting, some think, "By setting fasting times, I don't have to decide whether to eat or not each time." This is also psychologically important. If you have to decide "should I eat or should I endure" multiple times a day, decision-making itself becomes exhausting. By setting meal time rules in advance, the number of decisions decreases, making it easier to continue as a result.

On the other hand, cautious points should not be overlooked. Particularly, the question "Is intermittent fasting superior to regular calorie restriction?" is still under debate. A Cochrane review reported in 2026 examined clinical trials targeting overweight or obese adults and concluded that intermittent fasting does not clearly result in superior weight loss compared to standard dietary advice. In other words, while fasting may be an easier method for some people to continue, it does not guarantee magical, special weight loss effects.

So, what does this study teach us?

One thing is that we are moving from an era where weight loss success is measured solely by "how many kilograms were lost" to an era where we look at what is changing inside the body. While weight is an easy-to-understand indicator, behind it are brain reactions, gut microbiota, blood markers, appetite, and behavioral habits. To evaluate the value of intermittent fasting, it's necessary to consider not just weight differences but also the stability of eating behavior, blood sugar management, inflammation, sleep, and mental impact.

Another point is the large individual differences. Even with the same dietary method, it may be easier for some people to continue, while for others it may be stressful. On social media, there are positive voices like "increased concentration" and "easier on the stomach," while others say "irritated by hunger," "overeating as a rebound," and "it didn't suit me in the long run." This is not a contradiction, but a natural result given differences in constitution, life rhythm, work, sleep, medical history, and food culture.

Particularly noteworthy is that the dietary restriction used in this study was quite strict. Methods like setting days with 600 kcal for men and 500 kcal for women should not be easily imitated by the general public on their own. Fasting or extreme calorie restriction may pose risks for people with diabetes, hypoglycemia, a history of eating disorders, those who are pregnant or breastfeeding, in growth periods, have chronic illnesses, or are taking medication. It's advisable to consult healthcare providers or registered dietitians when practicing.

If we were to express the findings of this study in the simplest terms, it might be that "fasting times" do not merely empty the stomach but may change the conversation between the brain and gut microbiota. However, it's still unknown whether that conversation always works in a positive direction for health, how long it lasts, or who it is suitable for.

Information about dieting often tends to become extreme. There are claims like "fasting is the best" and counterarguments like "it's ultimately just about calories." However, what this study suggests is not either of those extremes. Calorie restriction is important, but human appetite and weight loss responses are based on a complex network created by the brain, gut, and microbes.

That's why future research should focus not just on "which dietary method results in the most weight loss," but on "which method works for whom and why." If we can predict who is more likely to succeed in weight loss, who is prone to rebound, and who is not suited for fasting based on gut microbiota patterns and brain activity characteristics, weight management may become more individualized.

Intermittent fasting is often discussed as a trendy diet method. However, this study places it in a deeper biological context. Creating hunger is not merely about endurance. It may have the potential to rearrange internal signals, change brain responses, and shake up the power dynamics of gut microbiota. Of course, there is still much to be proven. Nevertheless, it's likely that our bodies are changing more dynamically than we imagine behind the numbers on the scale.

Sources and References

ScienceDaily. An article published on May 31, 2026, providing an overview of how intermittent fasting-style dietary restrictions led to changes in brain activity and gut microbiota in obese adults.
https://www.sciencedaily.com/releases/2026/05/260530004622.htm

Original paper published in Frontiers in Cellular and Infection Microbiology. Primary information on research design, participants, fMRI, gut microbiota analysis, results, and limitations.
https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1269548/full

Example of reactions on Reddit. In the intermittent fasting community, voices share experiences of self-control and changes in cravings for sweets in response to a post introducing the same study, along with cautious points about the study's scale and long-term effects.
https://www.reddit.com/r/intermittentfasting/comments/18rzqhs/intermittent_fasting_seems_to_result_in_dynamic/

Search results from another Reddit thread. Discussions on fasting, the brain, and gut microbiota, with reactions regarding the differences from calorie restriction and how to interpret the research.
https://www.reddit.com/r/science/comments/18o03ff/how_fasting_makes_the_brain_and_gut_work_together/

Cochrane review on intermittent fasting. Background information that intermittent fasting does not clearly demonstrate superior weight loss effects compared to standard dietary advice.
https://www.cochrane.org/evidence/CD015610_intermittent-fasting-traditional-dietary-advice-or-no-treatment-which-works-better-help-adults